Specify wxGTK_VERSION explicitly for all Travis CI builds

It default to 2 currently, which is rather surprising, so prepare for
requiring specifying it explicitly by adding wxGTK_VERSION to all wxGTK
builds environments.

.travis.yml

@@ -36,11 +36,11 @@ jobs:
           name: wxGTK 3 CMake Ubuntu 18.04
         - dist: focal
           compiler: gcc
-          env: wxCONFIGURE_FLAGS="--disable-compat30 --disable-unicode" wxSKIP_SAMPLES=1
+          env: wxGTK_VERSION=2 wxCONFIGURE_FLAGS="--disable-compat30 --disable-unicode" wxSKIP_SAMPLES=1
           name: wxGTK ANSI Ubuntu 20.04
         - dist: focal
           compiler: gcc
-          env: wxCONFIGURE_FLAGS="--disable-compat30 --disable-sys-libs" wxSKIP_SAMPLES=1 wxUSE_ASAN=1
+          env: wxGTK_VERSION=3 wxCONFIGURE_FLAGS="--disable-compat30 --disable-sys-libs" wxSKIP_SAMPLES=1 wxUSE_ASAN=1
           name: wxGTK Ubuntu 20.04 with ASAN
         - os: osx
           osx_image: xcode7.3
@@ -82,7 +82,7 @@ jobs:
           name: wxQt Ubuntu 18.04
         - os: linux
           arch: arm64
-          env: wxCONFIGURE_FLAGS="--disable-sys-libs" wxLXC=1
+          env: wxGTK_VERSION=3 wxCONFIGURE_FLAGS="--disable-sys-libs" wxLXC=1
           name: wxGTK ARM64
         - os: linux
           arch: ppc64le
@@ -98,7 +98,7 @@ jobs:
     allow_failures:
         - os: linux
           arch: arm64
-          env: wxCONFIGURE_FLAGS="--disable-sys-libs" wxLXC=1
+          env: wxGTK_VERSION=3 wxCONFIGURE_FLAGS="--disable-sys-libs" wxLXC=1
           name: wxGTK ARM64
         - os: linux
           arch: ppc64le